Chapter 5 Least-cost paths
In this chapter we use the ‘least cost paths’ (a.k.a. LCP) methodology defined in Chapter 3 to define the path of least resistance from all coastal protected areas (larger than 1km2) in the focal region to their corresponding high elevation protected area. For each protected area (Section 1) and mangrove (Section 2), we calculate the LCP for the 10 nearest (by euclidean distance) high elevation protected areas, then select the path with the least resistance. We then explore several important properties of these paths between different countries (e.g. path length, mean resistance, total resistance, quality of forest on the path), in order to understand the spatial state of low-to-high elevation connectivity in central America. Finally, we devise a prioritization scheme to select candidate locations for habitat restoration initiatives.
It is important to note that the LCP method does not fully characterize connectivity in a probabilistic fashion, but it is a tool which cn give us simple information to design prioritization schemes. For a more complete consideration of connectivity, see the next chapter.
5.1 ‘End’ Nodes
We first start by defining a suite of end points (or nodes) - high elevation protected areas. To do this we import all of the high elevation protected area polygons defined in the data assembly section. As the LCP methods require points as targets, not polygons, we also randomly sample within the polygons to create a complete suite of “end nodes”. This means that there are multiple end points within larger protected areas.
In the map below, all of the end points are shown as black points, and their corresponding protected area polygons in blue:
In total, we have a suite (n = 1659) of end points for the analyses. For each protected area, we find the 10 closest of each of these end points (euclidian distance) and calculate the least cost paths to them.
5.2 Protected area climate corridors
We used the package ‘gDistance’ and the function ‘shortestPath’ to define the least-cost path from coastal protected areas larger than 1km2 in size to the ten nearest high elevation protected areas. Note, we use starting points adjacent to the ocean to fully encapsulate the ridge-to-reef gradients.
The map below shows the shortest (by distance) path (back lines) for each of the coastal protected areas. The underlying raster layer shows the cost surface, with red areas being highly resistant to movement, and blue areas offering low resistance.